Nostoc commune used in the study was collected from National Center for Conservation and Utilization of Blue-green Algae, Indian Agricultural Research Institute, New Delhi (India). The culture was grown in BG-11 growth medium (Stanier et al.,1971) containing (gL ^-1 ) MgSO_4.7H2O (0.75), NaNO₃ (1.5), CaCl₂.2H₂O (0.036), K₂HPO₄.3H₂O (0.04), Na₂ EDTA (0.001), Na₂CO₃ (0.02), Ferric ammonium citrate (0.006), citric acid (0.006) as macronutrients along with 1 ml of micronutrients (gL ^-1 ) H₃BO₃ (2.86), ZnSO₄.7H₂O (0.22), MnCl₂.4H₂O (1.81), CuSO₄.5H₂O (0.08), Na₂MoO₄.2H₂O (0.39) and CO(NO₃)₂.6H₂O (0.049) under rotatory conditions and illuminated continuously at a light intensity of 3500 LUX.

At the stationary phase of growth (25 days), the culture was harvested and spent media was collected after filtering the biomass. The collected biomass was dried in hot air oven at 60 ° C for one h and then placed in solvent. Left the mixture for few hours at room temperature and then sonicated for 10 min followed by centrifugation at 4000 rpm for 10 min. After centrifugation, supernatant was collected in a preweighed test tube and then concentrated under nitrogen until completely dry, again weighed and then resuspended in the appropriate solvent to make the solution of known concentration for the antibacterial assay. Same procedure was adopted for the preparation of aqueous extract.

The microorganisms used in antibacterial assay were collected from Institute of Microbial Technology (IMTECH), Chandigarh, India .The species employed include four Gram-positive bacteria (Staphylococcus aureus MTCC-740, Bacillus subtilis MTCC-736, Bacillus cereus MTCC-430, Bacillus pumilus MTCC-1607) and four Gram-negative bacteria (Escherichia coli MTCC-739, Pseudomonas aeruginosa MTCC-741, Salmonella typhi MTCC-733 and Klebsiella pneumoniae MTCC-139). The bacterial strain were inoculated on Tryptone Soya Agar (TSA) and incubated for 24 h at 30°C then suspended in saline solution 0.85% NaCl and adjusted to yield approximately 1.0 x 10 ⁸ -1.0 x 10 ⁹ cfu/ml by using spectrophotometer (25% transmittance at 530 nm). Media component were purchased from Hi Media, Mumbai, India. All the chemicals used were of analytical grade.

In vitro antibacterial activity of different crude extracts of Nostoc commune was evaluated using the agar well diffusion assay (Perez et al., 1990). 100 µl of adjusted culture was mixed with 100 ml of Muller Hinton Agar (MHA) and poured 25 ml each into sterile petridishes (90 mm), this was allowed to solidify and then individual plates were marked for the organisms inoculated. After solidification plates were punched to make the well of 6 mm diameter with the help of sterile cork borer. 100 µl of the respective cyanobacterial extracts were pipetted into the well in assay plates (Kaushik and Goyal, 2008). Plates were incubated overnight at 37 ^° C and all the plates were observed for the zone of inhibition, diameter of these zones were measured in millimeters. All the tests were performed under sterile conditions and repeated three times. The solvent control revealed no activity.

Minimum inhibitory concentration of active crude extract(s) was determined by broth microdilution method as recommended by National Committee for Clinical Laboratories Standards (NCCLS, 1997). The test was performed in 96 wells microtiter plates. Two fold serial dilutions of all active extracts were made in Cation-Adjusted Muller-Hinton Broth (CAMHB from Hi-Media) ranging from 1 to 512 µg/ml. Ciprofloxacin was used as standard antibiotic for the assay. Each inoculum was prepared in the same medium at a density adjusted to a 25% transmittance turbidity standard (10 ⁸ cfu/ml) and diluted to 1:100 for the assay. The final volume in the wells was 200 µl. After 24 h of incubation at 37°C, the calculated amount of nitrogen dried cyanobacterial material present in the most diluted extract that produced a visible inhibition was defined as MIC.

The absorption spectrum of methanol extract of Nostoc commune was recorded in the UV-Vis spectrophotometer (Shimadzu; Model No. UV-1700 Pharmaspec) capable of producing monochromatic light in the range of 200-800 nm for measuring the absorbance.

High performance liquid chromatography of methanol extract of Nostoc commune was performed on Shimadzu HPLC (Model No; 10 AVP) equipped with constant temperature column compartment, a sample injector capable of injecting 20 µl aliquots and a programmable variable photodiode array detector and an integrator. The C-18, 4.6 x 250 mm stainless steel column (waters) was used at room temperature. Extracts was eluted with mobile phase water:methanol (80:20) at the wavelength of 260 nm.